1. Field of the Invention
This invention relates to a surface potentiometer for measuring the surface potential of a surface to be measured. Specifically, it relates to a surface potentiometer suited for a measurement of the surface potential of a photosensitive medium in electrophotographic apparatus.
2. Description of the Prior Art
As a simple means for measuring a surface potential, use has heretofore been made of a method whereby a chopper blade biased to ground potential is rotated between a surface to be measured and a measurement electrode to convert DC potentials of the positive and the negative signs appearing on the surface to be measured at various timings into an alternating current and measure the same. However, when detecting a potential to be measured as an AC signal, the amplitude of the AC signal detected is proportional to a difference between the measured potential of the surface to be measured and the potential of the potentiometer housing, but whether the aforementioned measured potential is in the positive direction or in the negative direction with respect to the housing potential could not be discriminated. Particularly, there has been a problem where the surface potential measured is in both positive and negative directions as in the photosensitive drum of an electrophotographic apparatus.
To eliminate the above-noted disadvantage, it has been thought to discriminate between the positive and the negative of the surface potential by seizing the time when the chopper shields the measurement electrode and the surface to be measured. This is a very effective method, but it involves the necessity of providing the potentiometer with a detector element and circuit for discrimination, and the potentiometer itself has become complicated as well as the circuit.